Article

What is the pressure at sea level?

Standard atmospheric pressure at sea level is 29.92 inHg (1013.25 hPa) — the same as 101,325 Pa, 14.696 psi, 760 mmHg, or 1 atm. It is a defined reference value, not a reading: the “sea-level pressure” in a forecast is a real station’s reading adjusted to what it would be at sea level, which is why a barometer at altitude shows a much lower number.

The standard value in every unit

Hectopascals and millibars are the same unit under two names, and 1 atm is simply this value given its own name. Everything else is a conversion of the same quantity.

Values per the International Standard Atmosphere: NOAA JetStream, Wikipedia, Atmospheric pressure. The inHg figure is 29.9212 to four places; the bar figure is the millibar figure divided by 1,000.

So yes: air pressure at sea level is 14.7 psi, rounded from 14.696. And in pascals it is 101,325. This site converts with one fixed factor, 1 hPa = 0.02953 inHg, as the methodology sets out.

A standard, not an average

29.92 inHg (1013.25 hPa) is not what anyone measured on a typical day. It is the sea-level starting point of the International Standard Atmosphere, a model adopted by ICAO and laid down in ICAO Document 7488. The model pairs that sea-level pressure with a sea-level temperature of 15 °C (59 °F) and a temperature fall of 6.5 °C for every kilometer of height (Oxford Reference). It exists so engineers, pilots and meteorologists share one yardstick — above a set transition altitude, for instance, aircraft altimeters everywhere are set to 29.92 inHg regardless of local weather (Icarus Jet).

Real averages land close to it. In this site’s ten years of Open-Meteo ERA5 hourly reanalysis (2015–2024), the mean sea-level pressure of all 50 cities falls between 29.89 inHg (1012.3 hPa) in Phoenix and 30.08 inHg (1018.5 hPa) in Atlanta. Denver averages 29.97 inHg (1014.8 hPa); Miami, 30.03 inHg (1017.1 hPa). Every one of them sits inside the site’s normal zone of 29.68–30.12 inHg (1005–1020 hPa).

Note the word “sea-level” in all of those. The average pressure at Earth’s actual surface is lower — about 29.09 inHg (985 hPa), per Wikipedia — because much of the surface is not at sea level. That difference is the rest of this article.

What “sea-level pressure” means in a forecast

The pressure in a weather report is almost never a raw reading. The National Weather Service glossary defines sea-level pressure as the pressure at sea level at a given location, and adds that when it is observed at a station not at sea level — nearly all stations — “it is a correction of the station pressure to sea level.” The correction accounts for the standard fall of pressure with height and for the influence of temperature on it.

The reason is comparison. A high-altitude station has less air above it, so its raw reading is low whatever the weather is doing. NOAA describes the fix: convert each station’s reading to the value it would show if the instrument sat at sea level, so every station shares a common denominator. Without it, pressure maps would draw mountains, not weather systems. WeatherHawks makes the point with Colorado Springs: read uncorrected, its pressure on a clear day would look like a hurricane.

This is the number weather apps, TV forecasts and this site report. Every comparative figure here is sea-level-reduced, and the pressure zones on every city chart are drawn in sea-level values.

Station pressure vs. sea-level pressure

Station pressure — also called absolute pressure — is what the air actually presses on the instrument where it sits: the weight of the atmosphere from the instrument’s elevation upward (Mintaka reference guide). Sea-level pressure is that reading corrected down to sea level. At the coast the two are nearly identical; in the mountains they are far apart.

The rule of thumb near the surface is that pressure falls about 0.01 inHg for every 10 feet you climb — roughly 1 inHg per 1,000 feet, or 1 hPa per 8.5–9 m (Kestrel Instruments). The true relationship is exponential, not linear, so the rule drifts the higher you go.

What a barometer should read at altitude

The table applies the standard-atmosphere formula — p = 1013.25 × (1 − 0.0065h / 288.15)^5.255 hPa, with h in meters, the ICAO form documented on Wikipedia’s pressure altitude page — to each city’s elevation in this site’s data.

Standard-atmosphere approximations, not measurements. They assume the 15 °C model atmosphere; real station pressure moves with the weather and the temperature profile. Elevations from this site’s data; “below standard” is 1013.25 hPa minus the station figure.

Two things follow. First, a home barometer in Denver reading about 24.6 inHg (834 hPa) is normal for that elevation, not a record low. Second, the rule of thumb holds well at Denver’s 5,299 ft — it predicts a 5.3 inHg drop, and the formula gives 5.30 — but by Colorado Springs’ 6,010 ft it overshoots, predicting 6.0 against 5.95. That is the exponential curve showing through. Each city page lists its own ten-year mean station pressure beside the sea-level figures, so the Denver page gives you a reanalysis-based number to set against this approximation.

Why your barometer reads lower than the forecast

Because it is very likely showing station pressure and the forecast is showing sea-level pressure. Phone and watch barometers, and many home weather stations, typically display the raw pressure where they sit unless told otherwise. On the coast that makes almost no difference. In Denver the ten-year mean sea-level pressure is 29.97 inHg (1014.8 hPa), while a standard-atmosphere station reading is about 24.62 inHg (833.7 hPa) — a gap of roughly 5.35 inHg (181 hPa), about thirty times the 0.18 inHg (6 hPa) that counts as a swing day.

Two ways to compare like with like

Correct the device. If your barometer or app lets you enter your elevation or a sea-level offset, doing so converts its reading into a sea-level-equivalent you can set beside the forecast. Treat the result as approximate. The NWS correction also depends on temperature, so a fixed offset at a fixed elevation will not match the official figure every day.

Compare the trend instead. An uncorrected device still tells you whether pressure is rising or falling, and how fast — which is the part that carries weather information, and the part the city pages’ charts and outlook calendars are built on. Falling and rising barometric pressure covers what a change over hours means and how big a big one is.

To convert station pressure to sea-level pressure by hand, add back the elevation term: the “below standard by” column above, or about 1 inHg per 1,000 feet near the surface. It will put you in the right range, not on the official number.

Why this site’s Denver number can differ from your Denver barometer

Even after you correct a device, two things about this site’s figures are worth knowing, and they cut in the same direction at altitude.

First, sea-level reduction is a calculation, and reducing a high-elevation reading to sea level inflates its apparent variability, as the methodology and this site’s caveats note. Denver and Colorado Springs rank first and second of the 50 cities for swing days, partly for this reason. Denver’s standard deviation of sea-level pressure is 0.24 inHg (8.17 hPa), against a national median of 0.20 inHg (6.87 hPa) — though by that measure Denver ranks fifth, behind Salt Lake City at 0.26 inHg (8.64 hPa) and sea-level Boston at 0.25 inHg (8.44 hPa), which is why ‘partly’ matters.

Second, the historical figures are Open-Meteo ERA5 hourly reanalysis — a physically consistent model reconstruction constrained by observations, taken at each city’s centroid — not a barometer on anyone’s wall. Reanalysis makes cities directly comparable, at the cost of smoothing very local excursions. So a Denver reader’s corrected instrument and the Denver page can legitimately disagree by a little, and neither is broken. The methodology covers both caveats.

What to do with a sea-level number

Once you have a sea-level-equivalent reading, it can be read against the site’s five fixed pressure zones. Normal barometric pressure explains the 29.68–30.12 inHg (1005–1020 hPa) band around the standard, and low and high barometric pressure covers where the cut-offs fall either side. For pressure-sensitive readers, the site tracks change rather than level: pressure change has been statistically associated with migraine onset in some people and not in others. If your town is not one of the 50 city pages, check any US location.

Pressure change has been statistically associated with migraine onset in some people and not in others; effect sizes are modest and individual sensitivity varies widely. Nothing on this page predicts a headache, diagnoses anything, or is medical advice. If migraine is affecting your life, that is a conversation for a clinician.

All 50 city pages · Check any US location · Methodology